Radio receiving apparatus



March 31, 1942. R WEBER 2,278,030

RADIO RECEIVING APPARATUS Filed July 19, 1940 IN VENTOR.

V H .Ezwu'e f Weber Patented 31, 1942 RADIO RECEIVING APPARATUS RennieI. Weber, Franklin Park, 111., assignor to Zenith Radio Corporation,Chicago, 111., a corporation of Illinois Application July 19, 1940,Serial No. 346,271

. 2 Claims.

This invention relates to radio receiving apparatus, and moreparticularly concerns the provision of an untuned radio frequency stagetherefor comprising two or more parts, each of which parts comprises acircuit path resonant at a particular frequency, the resonantfrequencies of the various circuit paths being at difi'erent pointsalong a given frequency range or scale which is greater than andembraces the frequency tuning-flange of the apparatus.

She object of the invention is to eliminate some of the mechanicallyoperated parts heretofore required in radio apparatus for tuning,without reducing the efilciency, selectivity, or sensitivity of suchapparatus.

Another object is to obtain a relatively corrective gain at allfrequencies throughout the frequency tuning range of the radioapparatus, to thereby accomplish equal antenna sensitivity over theentire tuning range thereof.

Still another object is to reduce the intermediate frequency response inthe lower section of the tuning range of radio receiving apparatus.

A further object is to reduce the image interference throughout thetuning range of radio receiving apparatus, and particularly in the highfrequency section thereof, as it is in the upper region of the tuningrange where detrimentally high image interference is most oftenencountered.

A still further object is to obtain strong signal attenuation atfrequencies outside the tuning range of the apparatus.

Referring now to the accompanying drawing:

' Figure l is a circuit diagram of radio receiving apparatus embodyingmy invention, and

' Figure 2 is a slightly modified detail view of that portion of thecircuit diagram of Figure 1 which embodies my invention and which isdefined therein by dotted lines.

In radio receiving apparatus of the superheterodyne type, the signal isusually received through a tuned antenna stage, which is a circuit orstage that is selectively tunable to reso-' nance at the frequency ofthe carrier wave. of the desired signal, and in some superheterodynecircuit arrangements, the signal thus received is passed directly intothe modulator or converter stage, while in others, the signal impulsesare passed through a tuned radio frequency ampliflcation stage into themodulator or converter stage. The system of passing the received signalimpulses directly into the modulation stage has been foundunsatisfactory under many conditions. There is a strong tendency, forexample, for the intermediate frequency response signals to interferewith the reception of the desired signal. Another drawback has beenimage signal interference in the upper section of the frequency tuningrange of the apparatus. In order to overcome these difliculties, a stageof tuned radio frequency amplification has been employed between thefirst or antenna stage and the modulator or converter stage, but thissystem, while quite satisfactory in many respects, requires that theantenna stage and the radio frequency stage be tuned synchronously andthat both be tuned in synchronous relation with the oscillator circuit.

In the construction of compact receivers, the usual fixed arrangement ofthe rotors of the three variable condensers (which are commonly used inthe last mentioned system for tuning the antenna and radio frequencystages and the oscillator circuit) upon a common shaft to effect thenecessary synchronization during tuning creates a serious designlimitation, and where automatic or push-button tuning is provided as anauxiliary condenser-adJusting tuning means, the bulkiness of athree-gang condenser becomes an even more serious consideration. With myinvention of the untuned radio frequency stage, one of theabove-mentioned tuning condensers is no longer needed, and a greaterdesign freedom results therefrom.

With automatic or push-button tuning of the variabl inductance type, itis most desirable that the tuning be limited to the antenna stage andthe oscillator circuit. For example, if automatic tuning of this type isprovided to selectively cover five transmitting stations, it iscustomary to use five separate tuning units for tuning the antenna stageand five for tuning the oscillator circuit, and, if a radio frequencystage is to be tuned, five more tuning units are required. Theelimination of the five last-mentioned tuning units, through the use ofmy untuned radio frequency stage, adds another advantage in designing.

There are two sources of interference which most often affect the lowertuning rangefrequencies. Signals coming from stations transmitting at ornear the intermediate frequencyof the receiving apparatus is one source,and feedback of the intermediate frequency signal of the finalintermediate frequency amplifier of the apparatus itself is another. Asto the firstmentioned source, although the radio receiving apparatus isnot tuned at or near the aforesaid intermediate frequency, the apparatuswith only an antenna stage in the radio frequency section. or portionthereof will not, except under very favorable conditions, preventundesired signals coming from the aforesaid stations transmitting at ornear said intermediate frequency from interfering with the desiredsignal for which the apparatus is receptively tuned. As to thesecond-mentioned source, feedback of the intermediate frequency signalwill occur whenever there is an opportunity for a pickup of theintermediate frequency signal of the apparatus by the atenna stage. Theproblem of preventing the interference by such feedback of theintermediate frequency signal is most serious in the construction ofcompact-radio receivers, which include a loop antenna built into orsupported within the casing or cabinet thereof. The attenuation, or lackof gain, of my untuned radio frequency stage at frequencies below thetuning range of the apparatus blocks the passage of these interferingsignals to the modulator. or

' converter stage and thereby prevents interfersignal to which theapparatus is tuned.

ence with the desired signal.

Another type of interference is encountered when the receiving apparatusis tuned to a frequency in the upper section of its frequency tuningrange. This interference is created by an image signal which passesthrough the antenna stage to the converter and conflicts with the It iswell known that, if the receiving apparatus is tuned to 1200 kilocyclesand the intermediate frequency thereof is 455 kilocycles,. theoscillator frequency is 1655 kilocycles. If there should be a signalwhich is 455 kilocycles higher than the oscillator frequency, that is tosay 2110 kiloeycles, the' 2110 kilocycles signal will tend to passthrough the apparatus along with the 1200 kilocyeles signal, therebycreating serious interference. The herein described attenuation, or lackof gain, of my untuned radio frequency stage at frequencies above thefrequency tuning range of the apparatus effectively prevents the passingof the conflicting image signal into the converter or modulator stage.

It is desirable in radio receiving apparatus that, the gain orsensitivity throughout the tuning range thereof shallbe constant. Thatis to say, the gain at any one point in the frequency tuning rangeshould equal the gain at any other point therein. My untuned radiofrequency stage has this equal "gain characteristic within itself and issufliciently flexible to effect compensations for unevenness in gainthat may exist in other stages. If, for example, the gain or sens'tivityof the apparatus is low at a frequency of 700 kilocycles, thearrangement of my untuned radio frequency stage with compensatory gainat the same frequency of 700 kilocycles maintains the equal sensitivitythroughout the tuning range of the apparatus. Likewise, if a high gainobtains at a certain point in the frequency range in the other sectionsof the apparatus, it may be reduced by lowering the gain at that pointin my untuned radio frequency stage.

Figure 1 shows the essential circuits of a superheterodyne radioreceiver. The power supply circuit isnot shown. It will not be neces--sary to describe all the parts which are shown in Figure l, sncestandard electrical symbols have been applied. Reference numeral Iindicates the variable condenser of the antenna or first radio frequencystage, and reference numeral 2 the mediate signals.

variable condenser for the oscillator circuit. My untuned radiofrequency stage is defined by the dotted lines. The parts thereof shownin Figure 1 are resistance units 3' and l, inductance coils 5 and 6, andcondensers I, 8 and 9.

In Figure 2, my untuned'radio frequency stage is shown separated fromthe radio receiving apparatus. Also indicated, in dotted lines bystandard electrical symbols, are a number of so-called stray capacities.Because the location and value of these stray capacities may becontrolled, we may consider them for the purposes of this explanation asactual condensers placed as indicated in Figure 2. The arrangements andthe values of the units shown are for receiving apparatus with afrequency tuning range between 550 and 1600 kilocycles, but they shouldin no way be considered as limiting this invention,

since not only may a frequency tuning range or band be 'used indifferent positions within the complete range of radio frequencies, butthe outer limits of such frequency tuning range may be further separatedor moved more closely together.

The number of resonant circuit paths which constitute my untuned radiofrequency stage are two or more. In a given network of inductance units,resistance units, and capacitance units, the number of resonant circuitpaths which may be traced are only a few less than the total number ofseparate units in the network. For this reason, it is quite possible tofind more than the tour which will now be described. However, in fullyvdescribing the invention by this example, it is preferred to consider myuntuned radio frequency stage, as arranged in this form of theinvention, as consistingof four resonant circuit denser I, as shown inFigures 1 and 2. This path is usually designed for resonance at afrequency below the frequency tuning range of the apparatus. Thefrequency chosen for resonance is usually that of the intermediatefrequency, which in this example of the invention is 455 kilocycles.Asthe impedance of this series-connected circuit path is low, thevoltage is also low, and thus the sensitivity at the resonant frequencyis low. This low sensitivity, or low gain, at frequencies below thetuning range of the apparatus effectively blocks any interfering inter-The second path'to be considered consists of the inductance coil 5 andthe stray capacitance indicated by reference numeral II, the condenser Iand the stray capacitance II. The inductance coil 5 is in parallelconnection with the capacitances l0 and II and in series with condenserI.

The resonance of this path is usually placed within the lower half ofthe tuning range of the apparatus. Inthis example, the frequency is 600kilocycles and the tuning range of the apparatus is between 540 and 1600k-ilocycles. The impedance of this circuit path being relatively high, ahigh voltage results and thereby high gain.

Thethird path to be considered consists of inductance coil 0, condensersI and Q, and the stray capacitance It. In this circuit, inductance coil6 is in series connection with two parallel branches, one branchconsisting of condenser I and the other "branch consisting of condenser0 in series with the stray capacitance l2. The impedance of this circuitpath is low, but in this case the gain is high, since the output isconnected across the capacitive elements 8, 9 and i2, and is thus takenat a point of high voltage. The resonant frequencyis chosen to be withinthe upper half of the frequency tuning range of the apparatus.kilocycles.

The fourth resonant circuit path to be considered in this explanation ofmy untuned radio frequency stage consists of inductance coil and In thisparticular example, it is 1400 the distributed capacitance l3. Theresonant fre I quency is chosen to be above the tuning range of theapparatus. At frequencies above the tuning range of the apparatus, theimpedance is high and this impedance, being in series with the output,reducesthe gain. The resonant frequency of this circuit, as shown inthisparticular arrangement of my untuned radio frequency stage, is 2100kilocycles.

As may be seen from the description of the various resonant circuits,there is strong signal attenuation at frequencies below and above thetuning range, and relatively equal amplification throughout the tuningrange frequencies. There are two convenient means of controlling thegain throughout the tuning range. The first is the resistance unit 3.Changes in the resistance in this unit affect the gain throughout thefrequency tuning range, and particularly in the lower half thereof.Variations in the resistance of unit 3 varies the overall impedance ofthe path and will vary the voltage developed in the second de-' scribedcircuit path. The second control means is the condenser 8. Variation ofthe capacitance of this condenser will vary the developed voltage, andin this manner serve as a convenient means for controlling the gain inthe upper half of the frequency tuning range.

I claim:

1. In radio receiving apparatus, an untuned radio frequency transmissionstage comprising a radio frequency amplification tube and a plurality ofoutput circuit paths connected therewith and resonant at differentfrequencies, two of said circuit paths having relatively low gaincharacteristics and two of said circuit paths having relatively highgain characteristics, one of said relatively low gain circuit pathsextending between said tube and a radio frequency ground connection andanother of said relatively low gain circuit paths being locatedeffectively inthe output of said stage, and both of said relatively highgain circuit paths extending-between said tube and a radio frequencyground connection, the over-all frequency transmission range of saidstage and the tuning range of said apparatus lying between anddetermined by the frequencies at which the aforesaid relatively low gaincircuit paths are resonant, and the equalization of the signalamplification throughout the tuning range of said apparatus beingeffected through the employment of the aforesaid relatively high gaincircuit paths at frequencies within the frequency transmission range ofsaid stage.

2. In radio receiving apparatus, an untuned radio frequency transmissionstage comprising a radio frequency amplification tube and a plurality ofoutput circuit paths connected therewith and resonant at differentfrequencies, two of said circuit paths having relatively low gaincharacteristics and two of said circuit paths having relatively highgain characteristics, one of said relatively low gain circuit pathsextending between said tube and a radio frequency ground connection andanother of said relatively low gain circuit paths being locatedeffectively in the out put of said stage, and both of said relativelyhigh gain circuit paths extending between said tube and a radiofrequency ground connection, the over-all frequency transmission rangeof said stage and the tuning range of said apparatus lying between anddetermined by the positions occupied by the resonant frequencies of theaforesaid relatively low gain circuit paths in the radio frequencyspectrum, and the level of the amplification characteristics of saidstage and the equalization of thesignal amplification of said apparatusbeing effected by the positioning of the resonant frequencies of theaforesaid relatively high gain circuit paths in. the radio frequencyspectrum between the resonant frequencies of the aforesaid relativelylow gain circuit paths.

RENNIE I. .WEBER.

